Some dogs have little interest in toys (like my dog!) and have to be 'taught' how to play. Stuffing a Kong is a great way to get them interested, you can also freeze it with food inside to make the game harder.

We couldn't get Austin interested in balls until we tried one of those Kong squeaky tennis balls. He is still only interested in cuz toy that squeak, wubbas, and kong squeaky balls, and the flirt pole.

You said that the Doxie plays well. Is she overwhelming him? Or do you play with them separately? Just thinking because you said he was so submissive if your other dog could be making him nervous.... Playing treat seek and find games can help to boost a dogs confidence maybe if you paired finding a treat with finding a toy it could be fun for him.

It took Nugget six months from after I found him (at one year old +/-) before he even showed slight interest in toys. It wasn't until he was holding a toy and I pulled on it, and the tug instinct kicked in that I could see a lightbulb go off above his head. I quickly added some rules to the game (he is only allowed to take a toy when sitting, never jumping up, and he learned drop to take pauses during play), and it's his second favorite pastime after eating. New toys still need to be made "fun" (with a flirt pole or some tug-o-war game) but he is way more playful now than when I got him.